While it is common that, by having communications between a terminal (hereinafter refer to user apparatus UE) and a base station BS, the communication between user apparatuses UEs is performed in mobile communications, recently, various technologies have been studied on direct communications between the user apparatuses UEs.
It is necessary for a user apparatus UE to discover another nearby user apparatus UE when communications between the user apparatuses UEs are performed. The present application relates to the discovery of the user apparatus UE in the communications between the user apparatuses UEs.
As one of the methods for discovering a user apparatus UE, there is a method in which each user apparatus UE transmits (broadcasts) a discovery signal which includes its own identification information. For example, as shown in FIG. 1, in the case where a user apparatus UE-A transmits a discovery signal including its identification information and the discovery signal is received by a user apparatus UE-B, the UE-B discovers the user apparatus UE-A by determining that the discovery signal includes the identification information of the user apparatus UE-A. It should be noted that, as an example, the user apparatus UE-B, which has discovered the user apparatus UE-A, transmits a notice directly or via a base station BS to the user apparatus UE-A, indicating that the UE-B has discovered the UE-A, and communicates with the user apparatus UE-A.
Here, a distance from the user apparatus UE, within which another user apparatus UE can discover the user apparatus UE, depends on transmission power with which the user apparatus UE transmits the discovery signal.
FIG. 2 shows a situation in which the user apparatus UE-A transmits the discovery signal and user apparatuses UE-B through UE-D discover the user apparatus UE-A. As shown in FIG. 2, in the case where the transmission power of the user apparatus UE-A is low, the user apparatus UE-A is discovered by the UE-B only which is within a short distance from the UE-A. In the case where the transmission power of the user apparatus UE-A is medium, the user apparatus UE-A is discovered by the user apparatuses UE-B and UE-C. In the case where the transmission power of the user apparatus UE-A is high, the user apparatus UE-A is discovered by the user apparatuses UE-B, UE-C, and UE-D.
Although, as described above, the distance within which another user apparatus UE can discover the user apparatus UE can be determined according to the transmission power of the discovery signal transmitted by the user apparatus UE, the transmission power of the discovery signal transmitted by the user apparatus UE is not constant and varies depending on various cases shown below. It should be noted that the cases shown below are just examples.